Polysulfide-based resins are used widely in the protection and sealing of components and hardware comprising aluminum and aluminum alloys and also in providing sealing of enclosures containing a range of hydrocarbon-based fuels. In fact, polysulfide-based resins are the primary products of choice for sealing fuel tank compartments manufactured from aluminum and aluminum alloys. The aviation and automotive markets require fuel tank sealants, and are highly regulated by the federal government to provide specific performance qualities. These polysulfide sealant formulations have been held constant over many decades. The sealants are used in many aircraft, from small recreational, to commercial airlines, including the military.
When it becomes necessary to perform repair and maintenance on the polysulfide sealant, it is most common to begin the process with complete removal of the material from the area in question. Removal of the sealant typically includes scraping and mechanical abrasion, resulting in significant damage to the underlying substrate, typically composed of aluminum and aluminum alloys and many times are protected by chemical passivation or coated by cured epoxy. When substrate damage occurs, as is commonly the case with mechanical removal of the polysulfide sealant, the surfaces must be reconditioned, requiring the use of several steps that mechanically polish and chemically passivate the aluminum or aluminum alloy. Once the metal is passivated, there are further steps to prime the surface and coat with a cured epoxy resin. These process steps, each with a separate inspection, require an accumulation of time that can add hours onto a sealant integrity test within a defined area.
Alternatively, substrate damage by mechanical removal of polysulfide sealant may be avoided by the use of solvent mixtures through immersion or spray operations. When liquids are difficult or impossible to control their flow into select areas or dripping and spreading from vertical and overhead structures, the option exists to convert the liquid to a gel-form. Compositions of liquid and gel-form types of polysulfide removal are described in U.S. Pat. No. 7,005,409 (Dissolving Gel for Cured Polysulfide Resins, 2006), Moore et. al. The reacted polysulfide species in these forms may be washed away instead of wiping with a rag or napkin. Rinses include alcohol or water, which emulsify and disperse the reacted species and rapidly remove it from the surface, orifices, and cracks thereof, whereby continued rinsing will allow the substrate to be left clean.
Due to the aggressive nature of the liquid and gel-forms of the polysulfide sealant remover, it is also a common observance of the chemistry to attack certain metals by means of oxidation, corrosion, and the like, most specifically to aluminum metal and alloys thereof. When these liquid and gel-form cleaners are observed to attack the substrate, additional hours of corrective action must occur with a follow-on sealant integrity test. In such occurrences, the benefits of using a liquid or gel-form cleaner may be lost by the additional practices of correcting substrate damage. It is the object of this invention to provide compositions of matter, which are metal safe, and methods, which effectively and efficiently remove, cured polysulfide sealant.
Maintenance on underground storage tanks and aircraft must be performed regularly. In such cases, leak tests and other integrity screens are conducted. When it becomes necessary to undertake work to correct a problem, the tank or aircraft must be removed from service. This removal time imparts significant costs onto the system depending upon its service. For an underground tank, the business must close off this area from use for a period of time necessary to complete work. In the case of a service station and depending upon the extent of work, the business may have to completely close down in order to protect public safety. Following work completion, the tank may continue to be out of service until inspected and certified by government officials. Costs may approach double-digit percentages of the annual revenue for that service station.
In the case of an aircraft, polysulfide sealant inspection and maintenance may represent a scheduled review, which cause other active aircraft to be shuffled around to take its place. Engineers must schedule service at special locations suitable for such work and, depending upon the severity of the condition, a time estimate is assigned for completion, inspection, and certification of the work. In the case of certain airlines that contain a fleet of several hundred aircraft, this sort of timing may represent several days out of service, but be measured by the hour. What are worrisome are the delays, which occasionally occur due to multi-step polysulfide removal. An aircraft that misses its reactivation into the fleet will immediately force other active aircraft to be shuffled around to provide coverage for missed locations. This activity causes a far-reaching burden to the airline fleet and soaring costs, usually several factors multiplied by the price of the original service. For this reason and several others, a metal-safe chemistry is needed to remove polysulfide sealant in a manner that is mechanically and chemically safe for the substrate.
As a consequence of the needs for an effective and practical dissolution mechanism for polysulfide resin while exhibiting substrate metal safety, the composition of the present invention was developed and found to be effective. The metal-safe liquid penetrates and flows around the subject to be cleaned while the gel adheres well and allows the chemical formulation to be in direct contact with the cured polysulfide resin present on vertical and overhead surfaces. As polysulfide is reacted, it is dispersed to the bulk mixture, leaving underlying new resin to be exposed, reacted, and subsequently removed by the liquid. This cycle continues until all material is reacted and dispersed within the remover chemistry until the substrate is the last surface to be exposed to the liquid and occurs without any observed effects or damage. The process also occurs within the gel-form of a metal safe polysulfide remover. The process is accelerated by periodic removal by wiping of reacted chemistry and reapplying with fresh removal gel, until the substrate is exposed and protected using the metal safe invention.
The invention composition protects the substrate metal without sacrifice to the removal performance of polysulfide sealant using the form of a liquid or a gel. Although it is well known that inhibitors for corrosion or damage control may be simply added to formulations, this practice commonly results in a reduction in removal performance. Namely, the inhibitor not only inhibits corrosion to the substrate, however, it will affect the active ingredients in a way to reduce performance. This invention does not consider these simple approaches. Instead, the invention composition was designed in a manner to maintain high performance in removing polysulfide in both liquid and gel forms of the product.